Ecosystems (2006) 9: 63–74 DOI: 10.1007/s10021-004-0039-5

Coupled Nitrogen and Calcium Cycles in Forests of the Oregon Coast Range

Steven S. Perakis,1,2* Douglas A. Maguire,2 Thomas D. Bullen,3 Kermit Cromack,2 Richard H. Waring,2 and James R. Boyle2

1US Geological Survey, Forest and Rangeland Ecosystem Science Center, Corvallis, Oregon 97331, USA; 2Department of Forest Science, Oregon State University, Corvallis, Oregon 97333, USA; 3US Geological Survey, Branch of Regional Research, Water Resources Discipline, Menlo Park, California 94025, USA

ABSTRACT Nitrogen (N) is a critical limiting nutrient that concentrations are highly sensitive to variations in regulates plant productivity and the cycling of soil Ca across our sites. Natural abundance calcium other essential elements in forests. We measured isotopes (d44Ca) in exchangeable and acid leach- foliar and soil nutrients in 22 young Douglas-fir able pools of surface soil measured at a single site stands in the Oregon Coast Range to examine showed 1 per mil depletion relative to deep soil, patterns of nutrient availability across a gradient of suggesting strong Ca recycling to meet tree de- N-poor to N-rich soils. N in surface mineral soil mands. Overall, the biogeochemical response of ranged from 0.15 to 1.05% N, and was positively these Douglas-fir forests to gradients in soil N is related to a doubling of foliar N across sites. Foliar N similar to changes associated with chronic N in half of the sites exceeded 1.4% N, which is deposition in more polluted temperate regions, and considered above the threshold of N-limitation in raises the possibility that Ca may be deficient on coastal Oregon Douglas-fir. Available nitrate in- excessively N-rich sites. We conclude that wide creased five-fold across this gradient, whereas gradients in soil N can drive non-linear changes in exchangeable magnesium (Mg) and calcium (Ca) base-cation biogeochemistry, particularly as forests in soils declined, suggesting that nitrate leaching cross a threshold from N-limitation to N-saturation. influences base cation availability more than soil The most acute changes may occur in forests where parent material across our sites. Natural abundance base cations are derived principally from atmo- strontium isotopes (87Sr/86Sr) of a single site indi- spheric inputs. cated that 97% of available base cations can origi- nate from atmospheric inputs of marine aerosols, Key words: nitrogen cycle; N-saturation; base with negligible contributions from weathering. cations; calcium; magnesium; strontium; stable Low annual inputs of Ca relative to Douglas-fir isotopes; atmospheric deposition; marine seasalt growth requirements may explain why foliar Ca aerosols; nutrient limitation; Douglas-fir.

INTRODUCTION actions that impact the long-term health and sta- bility of forest ecosystems. Where plant growth is Biogeochemical couplings between nitrogen (N) limited by N availability, moderate increases in N and calcium (Ca) are an important suite of inter- supply can stimulate both plant growth and Ca uptake from soil. More dramatic increases in N Received 23 March 2004; accepted 10 November 2004; published online 30 January 2006. supply can overcome N limitation, drive forests *Corresponding author; e-mail: [email protected] towards N saturation, and accelerate coupled

63 64 S. S. Perakis and others leaching of N and Ca. In this way, long-term in- examine the long-term consequences of elevated N creases in N supply have a dual effect of stimulating on forest biogeochemistry. We here use a range of plant Ca demands and depleting Ca from soils. Douglas-fir forests in the Oregon Coast Range to These opposing drivers are thought to cause pro- ask the following questions: Do variations in soil N gressively non-linear N–Ca interactions along gra- status translate to changes in N availability for dients of ecosystem N status, yet such processes vegetation? How does base cation availability, in rarely are represented in forest biogeochemical particular Ca and Mg, change across a wide gradi- models (McNulty and others 1996; Asner and ent in soil N? Are N–Ca interactions linear or non- others 2001). linear across variations in N status? At a single There is broad recognition that chronic N depo- study site, we also use natural abundance 87Sr/86Sr sition is causing many temperate forests to cross a stable isotope ratios to identify long-term ecosys- critical threshold from N-limitation to N-saturation, tem sources of base cations, and introduce the use with serious implications for plant-soil Ca cycling of novel 44Ca/40Ca stable isotopes to evaluate (Aber and others 1998; McLaughlin and Wimmer pathways of Ca recycling in the plant-soil system. 1999). At the same time that N supply has in- creased, atmospheric Ca inputs have decreased in METHODS many areas (Hedin and others 1994). Preferential depletion of Ca relative to other elements by forest Site Description harvest is also of particular concern, especially in We conducted this research in young Douglas-fir N-rich areas (Federer and others 1989). These forests in the north-central Coast Range of Oregon. changes, combined with long-standing impacts Results from two collections are reported: (1) foliar from acid deposition, are modifying ecosystem Ca and soil nutrient concentrations across 22 sites balances and creating Ca deficiencies over large spanning a gradient of soil N availability, and (2) areas of temperate forest (Shortle and Smith 1988; strontium and calcium isotope analyses from a Lawrence and others 1997; Adams and others single stand with high N availability. All stands 2000; Huntington 2000; Driscoll and others 2001; studied were established as plantations, and none Bailey and others 2004). It remains unclear from have been fertilized. Climate across the study area these studies, however, whether changes in eco- is temperate and maritime, with wet cool winters, system Ca cycling can be traced primarily to effects dry warm summers, and approximately 170 cm of excess N alone, and whether such changes will annual precipitation. intensify as long-term N inputs increase site N The 22 stands representing the N gradient were status. selected from a set being monitored to examine the Forests of the Pacific Northwest are far less im- effect of Swiss needle cast on Douglas-fir growth pacted by chronic N and other air pollutants than (Maguire and others 2002). Stands ranged from 11 many other temperate regions. These ecosystems to 25 years of age, and exhibited a range of needle are often considered N-poor and base-rich (for longevity from 0.8 to 3.6 years (Table 1). Soils are example, Sollins and others 1980; Cole and Gessel derived from either sedimentary (that is, sand- 1992; Fenn and others 2003). However, high rates stones and siltstones) or basalt parent material, of N input can occur in forests of the Pacific with Andic properties and silty to silty clay loam Northwest via biological N fixation associated with textures (NRCS, in press). Most study sites were red alder (100–200 kg N ha)1 y)1, Binkley and relatively flat and located on broad upland topo- others 1994). Fixed N contributes to exceptionally graphy, though a few sites may have received up- high soil N capital (up to 30,000 kg N ha)1, land colluvial inputs. The stand sampled for Remillard 2000), and can accelerate N cycling, strontium and calcium isotopes supported 45-year- nitrification, and Ca leaching in a manner similar to old Douglas-fir along a ridgeline with sandstone- chronic N pollution (Van Miegroet and Cole 1984). derived soils. These factors are thought to predispose coastal Douglas-fir forests to intensification of an endemic Sample Collection and Analysis fungal needle cast disease, resulting in 22–53% reductions in tree growth over the past decade, Samples of foliage and soil were collected across the particularly on N-rich sites (Waring and others N gradient in spring 2000. Sampling was based on 2000; Maguire and others 2002; El-Hajj and others five randomly selected target trees per site. Mineral 2004). soil (0–10 cm) at two locations within 5 m of each Exceptionally wide gradients of soil N in the target tree was collected using a 2 cm diameter Pacific Northwest provide a unique opportunity to corer, and all samples were combined to yield one Coupled Nitrogen and Calcium Cycles 65

Table 1. Site and Stand Characteristics of the Twenty-two Sites at the Time of Sampling in 2000

Site Mean Standing Annual Soil suborder Elevation Slope Stand age index needle volume incrementà Site (USDA)* Aspect (m) (%) (y) (m) age– (y) (m3/ha) (m3 ha)1y)1)

Sedimentary Music road Alic Hapludand N 484 5 16 47 3.3 38 11.0 7 Andic Dystrudept N 278 50 25 43 3.5 141 18.0 5 Andic Dystrudept SW 322 55 23 40 3.0 176 23.7 77 Andic Dystrudept W 235 10 22 45 3.0 221 25.9 58 Andic Dystrudept E 80 15 17 45 3.1 102 22.1 85 Typic Fulvudand SW 121 25 12 38 1.9 211 11.3 Juno Hill Andic Dystrudept S 86 10 18 36 1.1 61 4.5 16 Andic Dystrudept SW 115 15 20 38 1.7 89 10.5 Smith Creek Typic Fulvudand S 167 20 17 34 1.9 77 16.3 39 Andic Dystrudept S 23 10 21 45 1.8 123 16.3 Cloverdale Typic Fulvudand E 105 10 16 30 0.8 129 4.2 Basalt Steinburger Andic Dystrudept E 539 22 – 42 2.9 62 16.1 Jensen Typic Palehumult W 361 25 11 42 3.4 24 7.8 Hoag pass Alic Hapludand NW 755 20 12 30 3.6 27 60 Typic Fulvudand N 201 75 22 40 3.2 130 21.6 East Beaver Typic Fulvudand S 127 57 20 36 1.8 137 13.9 108 Alic Hapludand W 269 40 16 42 2.9 98 16.7 Chopping block Andic Dystrudept W 76 5 19 35 1.7 88 8.4 Powerline road Typic Fulvudand NW 78 47 17 36 1.8 92 15.7 51 Typic Fulvudand N 171 85 16 32 1.5 9 1.7 101 Typic Fulvudand SW 262 15 24 31 1.8 31 3.3 119 Typic Fulvudand S 366 30 18 13 0.9 2 0.5

Divided into sedimentary versus basalt soil parent material, ordered by increasing soil % N. *Provisional soil classifications based on ongoing revision of Tillamook and Benton County NRCS Soil Surveys. –Determined in 1999 from one branch in each third of the crown per tree, averaged over ten trees per site. àAverage annual increment for the 4 year period 1998–2002, except for 2 year periods at Cloverdale (1995–1996) and Plot 108 (1998–1999).

composite soil sample per site. The tip of the south- analyzed on a Leco-CNS 2000. Total elemental P, K, most branch in the fifth whorl of each tree was Ca, Fe, Mg, Mn, Cu, B, and Zn were determined by clipped off with a pole pruner, and all lateral 1999 microwave digestion with HNO3, followed by shoots were clipped and combined into one com- analysis on Perkin Elmer Optima 3000 DV ICP. + ) posite foliage sample per site. NH4 ,NO3 , and P in soil extracts were analyzed by Soil and foliage samples were analyzed by the Alpkem RFA 300 automated colorimetry. Central Analytical Laboratory at Oregon State The 45-year-old Douglas-fir stand was sampled University. Soil samples were sieved (2 mm) before in summer 2001 for natural abundance strontium analysis. Soil pH was determined in 1:2 mixture of and calcium isotopes in stemwood and soil. This soil:water, and a subsample of fresh soil was dried at stand was selected on the basis of displaying slow 105°C for 48 h to determine moisture content. rates of stand growth determined as part of a re- + ) Available NH4 and NO3 were assayed by extract- gional tree growth survey (Maguire and others ing 20 g soil with 75 mL 2 M KCl for 1 h. 2002). A single tree core taken to the pith at breast Exchangeable Ca, Mg, and K were assayed by height (1.4 m) was dried and separated into 5 year extracting 2 g soil with 40 mL of 1 N NH4OAc. growth increments using a dissecting scope. Sam- Available Bray-P was determined by extracting 2 g ples were digested in 5 mL of concentrated Teflon- soil with 20 mL of 0.03 N NH4F and 0.025 N HCl. All distilled HNO3 for 24 h, followed by evaporation of suspensions were filtered through Whatman 42 HNO3, treatment with concentrated HNO3 and filters. Soil and foliage samples were dried at 80°C Ultrex H2O2 to remove organic compounds, redis- and ground to fine powder prior to total elemental solution in 3 mL of 2% HNO3, and the final solu- analysis. Total C, N, and S in plant and soils were tion reserved for analysis. We collected litter and 66 S. S. Perakis and others soil immediately beneath the target tree. Litter was Calculations and Statistics collected to the surface of the mineral soil horizon. The 87Sr/86Sr isotopic composition of weathering Mineral soil was collected incrementally to the was calculated by plotting 87Sr/86Sr of the digestible depth of resistance using a 4 cm diameter corer (0– fraction in the two deepest soil samples against 5, 5–10, 10–15, 15–25, 25–35, 35–45, 45–55, and 87 86 55–65 cm). These soils received a three-stage Nb/Sr, with weathered Sr/ Sr estimated by linear extrapolation to the Nb-free end-member (Bullen sequential treatment, yielding exchangeable, and others 1999). Relationships between nutrient leachable, and digestible fractions (compare, Blum concentrations in soils and foliage were examined and others 2002; TD. Bullen and SW. Bailey, using Pearson correlations with Bonferroni adjusted 2005). The exchangeable fraction was assayed by -values. To illustrate graphically the response equilibrating 5 g of soil with 50 mL of 1 N NH OAc P 4 patterns for Ca and Mg in foliage and soil samples as for 24 h. The leachable fraction was assayed by thoroughly rinsing 1 g of soil remaining from the a function of soil N, we also developed non-linear exchange procedure with de-ionized water and regressions of the general form y = a + b/x. How- ever, we do not evaluate the statistical properties of combining with 10 mL of 1 N HNO for 24 h at 3 these regressions, because low values of X in 30°C. The digestible fraction was determined by equations of this form correspond to a wide range of ashing at 650°C, digesting in a mixture of con- possible Y values, resulting in intrinsically poor fit centrated HF (5 mL) and HNO (1 mL) at 30°C, 3 between and variables. All statistical tests were evaporating, redissolving in 10 mL of 6 N HCl, and X Y performed using SYSTAT 10.2 (SPSS, Chicago, IL), finally reevaporating and treating with HNO and 3 and were considered significant at a = 0.05. Ultrex H2O2 to eradicate organic compounds. Solutions were analyzed for major and trace elements on a Perkin Elmer Elan 6000 induc- tively-coupled plasma mass spectrometer. All tree RESULTS and soil samples were analyzed for Mg, Ca, Sr Soil Nutrients and Ba. Digestible fractions were analyzed for Nb, an immobile trace element used here to estimate Chemical properties of 0–10 cm mineral soil for 22 the isotopic composition of weathered 87Sr/86Sr. sites across the N gradient are listed in Table 2. Soil Sr and Ca were separated for isotopic analysis N ranged from 0.15 to 1.05% N across sites and + correlated with soil C (r = 0.96). Extractable NH4 from sample digests using AG50X8 cation resin ) with 2 N HCl eluant. Prior to Ca purification, an and NO3 exhibited saturating log-normal rela- aliquot of each sample sufficient to provide 750 tionships with soil N (Figure 1, bottom). ng of Ca was mixed with a calibrated 42Ca–48Ca Exchangeable calcium (r = )0.53) and magnesium double spike solution. The double spike allows (r = )0.42) displayed significant inverse variations correction for both instrumental mass discrimi- with soil N (Figure 2, bottom). Total S and N in nation and possible isotope fractionation associ- soils were positively correlated (r = 0.95). Soil N ated with non-quantitative recovery during was negatively related to pH (r = )0.73) and column chemistry. Both Sr and Ca isotopic available K (r = )0.45), and was unrelated to compositions were measured on a Finnigan MAT available P (r = 0.04). 261 thermal-ionization mass spectrometer. Inter- There were no significant differences in pH, C, nal precision of 87Sr/86Sr measurements was or soil nutrient concentrations between soils de- ±0.00003 or better at the 95% confidence level rived from basalt versus sedimentary parent (2r). For the Ca isotope measurements, the materials (t-tests, P = 0.12 for pH, P > 0.26 for all double spike procedure allows 44Ca/40Ca to be nutrients, n = 11 for each parent material). Aver- determined with an internal precision of ±0.15& age soil C and N concentrations, C:N ratio, and pH or better at the 2r level. Here we report the Ca were within 5% when comparing across parent isotopes as d44Ca, calculated as the per mil dif- material. Soils derived from basalt contained on ference of 44Ca/40Ca in the sample from that of average 63% more extractable Ca and Mg, and modern seawater. Seawater analyzed in our lab- 72% more Bray-P, but none of these differences oratory has 44Ca/40Ca of 0.021713. Further details were significant. of column chemistry, mass spectrometric tech- niques and data reduction procedures for the Ca Plant Nutrients isotope measurements are available elsewhere (Skulan and others 1997; Skulan and DePaolo Nutrient concentrations in Douglas-fir foliage 1999). across the N gradient are listed in Table 3. Foliar N Coupled Nitrogen and Calcium Cycles 67

Table 2. Soil Chemistry across 22 Douglas-fir Stands

+ ) C N S NH4 –N NO3 –N P K Ca Mg Site pH (%) (%) (%) C:N (lg/g) (lg/g) (lg/g) (lg/g) (meq/100 g) (meq/100 g)

Sedimentary Music road 5.4 5.84 0.21 0.02 28 4.1 0.9 5 137 1.4 0.5 7 5.8 5.94 0.24 0.01 25 4.3 0.8 7 335 4.9 2.8 5 5.7 5.67 0.26 0.02 22 5.1 1.8 8 296 5.6 3.8 77 5.5 5.14 0.30 0.01 17 6.2 3.2 19 328 5.1 3.2 58 5.0 7.40 0.37 0.03 20 8.5 5.0 7 218 1.3 0.9 85 5.2 8.81 0.48 0.05 18 10.4 3.7 7 207 0.8 1.0 Juno Hill 4.5 7.31 0.51 0.04 14 4.6 11.5 4 133 0.4 0.4 16 4.8 8.85 0.56 0.05 16 8.9 4.7 9 285 0.9 1.0 Smith Creek 5.0 11.70 0.61 0.06 19 7.2 8.7 3 149 1.3 1.3 39 4.3 16.50 0.69 0.06 24 10.2 9.5 6 218 1.7 1.6 Cloverdale 4.8 14.60 0.76 0.08 19 7.1 6.8 10 140 0.5 0.7 Basalt Steinburger 5.6 3.80 0.15 0.01 25 4.0 0.7 5 192 2.7 1.3 Jensen 6.1 4.93 0.18 0.01 27 4.4 0.5 7 386 6.2 3.3 Hoag pass 5.5 4.52 0.18 0.01 25 3.8 0.7 34 188 2.4 1.0 60 5.6 4.72 0.25 0.02 19 4.8 4.4 5 125 10.4 10.0 East Beaver 5.4 4.77 0.30 0.03 16 6.5 3.7 15 261 3.9 3.6 108 5.6 7.36 0.39 0.02 19 5.1 4.6 7 250 8.4 4.8 Chopping block 5.0 7.93 0.48 0.06 17 7.8 8.2 13 140 1.3 1.1 Powerline road 4.9 9.86 0.52 0.07 19 5.4 9.9 6 137 1.1 0.8 51 5.0 10.60 0.66 0.07 16 9.0 10.3 28 179 0.8 1.0 101 4.7 19.30 1.05 0.09 18 13.4 9.9 20 133 0.6 0.4 119 5.0 18.70 1.05 0.11 18 6.9 6.7 6 152 1.3 0.7

ranged from 0.85 to 1.74% N across sites, and in- mate that exchangeable Sr to 65 cm depth creased with soil N (Figure 1, top). Foliar C:N originates almost exclusively (>97%) from atmo- (average = 39, range = 30–61) was negatively re- spheric sources. A limited number of Ca isotope lated to soil N (r = )0.78). Significant correlations analyses show progressive depletion in d44Ca of between foliar N and other nutrients were observed exchangeable and leachable pools towards shallow for Ca (r = )0.67), S (r = 0.78), Mn (r = )0.60), soils, with values in 0–5 cm soil depth approxi- B(r = )0.62), and Zn (r = )0.46). Foliar Mg mately 1& depleted relative to Ca in HF digested (Figure 2, top) and foliar P, K, Fe, Cu did not vary residue (Figure 4). significantly with foliar N. 87Sr/86Sr in Douglas-fir stemwood displayed a narrow range of values (range: 0.70923–0.70946, Strontium and Calcium Isotopes mean = 0.70934, n = 8) similar to soil exchange- able pools (Figure 5, triangles). Using Sr atmo- Exchangeable 87Sr/86Sr in soil at the 45-year-old spheric and weathering end-members as above, we Douglas-fir site exhibited a narrow range of values estimate that more than 99% of 87Sr/86Sr in wood (Figure 3). Exchangeable 87Sr/86Sr ranged from is derived from atmospheric inputs over stand 0.7094 to 0.7096, with the exception of one value development (Figure 5). Ca/Sr of stemwood in- at 35–45 cm depth (87Sr/86Sr = 0.7086). 87Sr/86Sr creases steadily from 48 to 121 over the growth of soil HF digests ranged from 0.7148 at the surface history of the tree, compared to Ca/Sr equal to 120 to 0.7165 at depth. Based on a plot of 87Sr/86Sr for the exchangeable fraction of 0–5 cm soil, and against Nb/Sr for HF digests of the two deepest Ca/Sr equal to 162 for current leaf litter. Sr/Ba of all samples, we calculate 87Sr/86Sr of weathering as stemwood age classes (mean = 0.33, range 0.27– 0.7217 (Bullen and others 1999). From two-end 0.38, n = 8) was similar to that of the exchangeable member mixing calculations that set atmospheric fraction of 0–5 cm soil (Sr/Ba = 0.34). Sr/Ba de- inputs to seawater values (0.7092) and set weath- clined logarithmically (r2 = 0.93, P < 0.001) in ering to our calculated 87Sr/86Sr (0.7217), we esti- deeper soil samples to a value of 0.02 at 60 cm. 68 S. S. Perakis and others

Figure 2. Top Panel Foliar Ca (solid symbols) and Mg Figure 1. Top Panel Foliar N plotted against soil N in 0–10 (hollow symbols) plotted against soil N across 22 Douglas- cm mineral soil for 22 Douglas-fir forests. Circles indicate fir forests. Circles indicate soils derived from sedimentary soils derived from sedimentary parent materials, triangles parent materials, triangles indicate basalt derived soils. indicate basalt derived soils. Line includes all points, Bottom Panel Soil exchangeable Ca and Mg, symbols as 2 y = 0.34 ln(x) + 1.69, r = 0.73, P < 0.001. Bottom Panel above. For both panels, we show regression lines of the + ) Extractable NH4 (hollow symbols) and NO3 (solid symbols) general form y = a + b/x to illustrate response patterns. plotted against soil N. Regression lines include both bed- Significant Pearson’s correlations (P < 0.05) with soil N + rock types for NH4 = 3.3 ln(x) + 9.7 (dashed line, are calculated for foliar Ca (r = )0.61), soil Ca 2 ) r = 0.55, P < 0.001), and NO3 = 5.3 ln(x) + 10.1 (solid (r = )0.53), and soil Mg (r = )0.42). line, r2 = 0.69, P < 0.001).

DISCUSSION limiting. However, this feedback does not continue indefinitely to create a ‘‘runaway’’ N cycle, as Gradient of Nitrogen Biogeochemistry indicated by the asymptotic behavior of both foliar We found an approximately ten fold gradient in and soil inorganic N in very N-rich sites (Figure 1). surface soil N concentrations across the Oregon Selection against luxury N uptake in plants to Coast Range, ranging from low values (0.15% N) minimize herbivore and pathogen risks (Chapin typical of N-poor temperate forests to very high and others 2002), or acceleration of N losses to limit values (1.05% N) characteristic of N-rich tropical inorganic N accumulation in soils (Perakis 2002; + ) forests. Soil inorganic N (NH4 +NO3 ) and foliar N Perakis and others 2005), may constrain plant-soil increased markedly across this soil N gradient, feedbacks when N is not limiting. ) consistent with a positive feedback cycle of plant Coupled leaching losses of NO3 and base cations nutrient use that reinforces local patterns of nutri- are likely to explain the inverse relationship we ent availability (Vitousek 1982; Prescott and others observed between soil N and exchangeable Ca and 2000). Foliar N in half of the sites exceeded 1.4% N, Mg in soils (Figure 2). This conclusion is supported which is considered above the threshold of N-lim- by watershed-level patterns of nutrient export ) itation in coastal Oregon Douglas-fir (Hopmans and across our study area. NO3 dominates anion fluxes Chappell 1994). This suggests that positive rein- in most Coast Range streams, and is closely corre- forcement of plant-soil N cycling feedbacks can lated with Ca and Mg levels, which together persist beyond the point at which N is no longer account for more than 75% of total cation export in Coupled Nitrogen and Calcium Cycles 69

Table 3. Foliar Chemistry across 22 Douglas-fir Stands

C N S P K Ca Mg Mn Fe Cu B Zn Site (%) (%) (%) C:N (%) (%) (%) (%) (lg/g) (lg/g) (lg/g) (lg/g) (lg/g)

Sedimentary Music road 51.60 1.15 0.10 45 0.18 0.49 0.48 0.11 423 66 4 21 16 7 52.10 1.24 0.10 42 0.20 0.58 0.37 0.12 478 54 5 20 17 5 52.30 1.11 0.08 47 0.11 0.59 0.36 0.10 129 43 15 11 16 77 51.00 1.31 0.10 39 0.15 0.48 0.54 0.11 351 334 5 15 15 58 52.00 1.38 0.10 38 0.14 0.67 0.43 0.11 289 102 5 17 15 85 52.90 1.51 0.11 35 0.16 0.73 0.22 0.13 142 241 5 15 13 Juno Hill 53.30 1.53 0.11 35 0.14 0.57 0.16 0.12 226 79 4 14 12 16 52.70 1.61 0.11 33 0.14 0.73 0.23 0.11 197 384 5 13 13 Smith Creek 52.10 1.52 0.11 34 0.18 0.58 0.35 0.13 274 125 5 15 13 39 52.30 1.46 0.11 36 0.14 0.73 0.27 0.11 548 77 6 18 16 Cloverdale 53.10 1.59 0.12 33 0.13 0.52 0.17 0.17 328 79 5 10 16 Basalt Steinburger 51.50 0.85 0.09 61 0.23 0.65 0.59 0.13 810 62 4 23 16 Jensen 51.60 1.13 0.11 46 0.12 0.54 0.47 0.12 682 88 3 19 19 Hoag pass 51.20 1.15 0.09 45 0.21 0.48 0.24 0.10 382 68 3 11 13 60 52.00 1.20 0.10 43 0.14 0.49 0.58 0.18 215 83 4 11 11 East Beaver 52.80 1.58 0.11 33 0.19 0.67 0.37 0.21 168 112 4 11 11 108 51.60 1.28 0.10 40 0.20 0.77 0.53 0.14 211 428 6 16 16 Chopping block 53.40 1.61 0.11 33 0.17 0.66 0.28 0.15 118 284 5 12 13 Powerline road 52.10 1.23 0.10 42 0.14 0.64 0.28 0.16 202 134 4 15 14 51 53.30 1.55 0.11 34 0.20 0.73 0.25 0.16 202 219 5 13 13 101 53.70 1.59 0.10 34 0.17 0.54 0.25 0.15 190 120 4 11 14 119 52.70 1.74 0.12 30 0.15 0.41 0.28 0.12 248 64 4 7 14 excess of marine seasalt inputs (Compton and Atmospheric Constraints on Calcium others 2003). The strong inverse relationship that Availability we observed between soil N and base cations was 87 86 surprisingly robust to variations in soil parent Sr/ Sr ratios provide insight into long-term material across the study area, which raises the sources of base cations for forest ecosystems, par- ) ticularly Ca (Capo and others 1998). Using possibility that excess NO3 leaching – not geology 87 86 – is a primary control of surface soil Ca and Mg in Sr/ Sr ratios measured at a single study site, we these forests. found that more than 97% of base cations in plants How do such wide gradients in soil N develop in and soil could be traced to atmospheric inputs of a region of low N deposition? Large areas of the marine sea-salt aerosols (Figures 3,5). Atmospheric Oregon Coast Range that currently support conifer deposition dominates ecosystem Sr pools in other forests, including our study area, were dominated humid tropical and temperate coastal forests historically by the N-fixing tree red alder (Alnus (Kennnedy and others 1998, 2002; Poszwa and rubra) (RSH. Kennedy and TA. Spies, 2005). This others 2002), yet we were surprised to reproduce aggressive pioneer can fix 150 kg N ha)1 y)1 for this result in the Oregon Coast Range, which is many years before succession to more shade-tol- considered geologically dynamic, with active soil erant conifers (Binkley and others 1994). Prior to turnover by uplift, erosion, and landslides European settlement in the 1800’s, stand replacing (Lancaster and others 2001). In addition, high fires occurred at an average return interval of 200 watershed fluxes of base cations to streams suggest years (Long and others 1998), and were the most ample supplies of weatherable minerals in soils likely control over red alder distribution across the (Compton and others 2003). On the other hand, landscape. Variation in the extent and intensity of the old age of Coast Range parent materials (>30 Coast Range fires could thus shape local N balances m.y.b.p.) creates opportunities for long-term by controlling both the distribution of red alder N- weathering and local depletion of surface soil fixation, as well as fire induced N-losses from or- minerals, particularly on sites with low potential ganic N combustion. for erosion or colluvial inputs, where soils can be 70 S. S. Perakis and others

Figure 4. Calcium isotopes reported as d44Ca relative to seawater in soil pools from three depths, same site as Strontium isotope (87Sr/86Sr) composition of Figure 3. Figure 3. Squares denote NH4OAc fraction, circles denote exchangeable soil pools to 65 cm depth in a 45-year-old HNO3 leachable fraction, and triangles denote residual Douglas-fir forest. Dashed lines indicate expected Sr iso- HF digest fraction. The standard error of two laboratory tope composition of atmospheric versus weathering replicates is shown for all samples. Error bars are smaller inputs, as described in text. than symbols when not visible. several meters deep atop a thick (>5 m) mantle of through recycled nutrients from detritus. Alterna- weathered sandstone (Anderson and Dietrich 2001; tive explanations for directional changes in Ca/Sr Heimsath and others 2001). The ridgeline where ratios, such as shifts in tree foraging among differ- our study site was sampled for Sr isotopes may ent soil minerals (Blum and others 2002) or depths represent a stable landform with greater depletion (Poszwa and others 2002) over time, are unlikely of weatherable minerals relative to lower slope given constant Sr/Ba ratios recorded in stemwood positions. We expect that a broader survey of sites over this same period (data not shown). Overall, and landscape positions might reveal a range of the remarkable similarity between 87Sr/86Sr in atmospheric versus weathering source contribu- marine aerosols (0.7092), stemwood (0.7093), and tions of Sr (for example, Vitousek and others 2003). depth-integrated soil (0.7094) suggests strong reli- Nevertheless, the narrow range of Sr isotope values ance on a pool of accumulated atmospheric Sr, and that we measured in vegetation and soils by inference Ca. (range = 0.7092–0.7096, with a single value Our data suggests that Ca is far more susceptible 0.7086) so closely resembles seawater sources of than Mg to depletion at high soil N. Both marine aerosols (0.7092) that we are confident of a exchangeable Ca and Mg declined at high soil N, dominant role for atmospheric inputs in supplying yet only foliar Ca tracked these changes (Figure 2). base cations to our sample site. Foliar and soil base cations across sites were closely 87Sr/86Sr ratios preserved in Douglas-fir stem- correlated for Ca (r = 0.73, P < 0.001), but not for wood provide a retrospective record of Sr sources to Mg (r = 0.32, P = 0.86). Foliar Ca concentrations vegetation, and confirm an important long-term were at the low-end of values reported for field role for atmospheric inputs. 87Sr/86Sr in 5-year ra- grown Douglas-fir in the Pacific Northwest, dial increments of our sample tree indicates con- whereas foliar Mg was well within regional aver- sistent reliance (99–100%) on marine aerosol Sr ages (Walker and Gessel 1991). Ratios of Ca:Mg in throughout 45 years of growth (Figure 5). A pro- foliage from our sites declined from 5:1 to 1:1 as soil gressive increase in Ca/Sr of stemwood over this N increased, yet ratios less than 2 are extremely same period, plus elevated Ca/Sr in leaf litter, are uncommon regionally (calculated from Walker and consistent with preferential biological cycling of Ca Gessel 1991). Ca–Mg antagonism cannot explain relative to Sr (Poszwa and others 2000, 2002; the pattern of declining foliar Ca at high N, because Watmough and Dillon 2003), particularly because both elements were present at similar concentra- a large fraction of annual Ca uptake by trees occurs tions in exchangeable soil pools. Coupled Nitrogen and Calcium Cycles 71

Figure 5. Strontium isotopes (triangles) and Ca/Sr ratios (circles) in 5-year annual increments of stemwood of a 45 year-old Douglas-fir, same site as Figure 3. Square denotes Ca/Sr ratio of litter. Solid line is atmospheric end-member, set to 87Sr/86Sr of sea water (0.7092). Arrow indicates calculated 87Sr/86Sr weathering end-member (see text for details).

Disparities in rates of atmospheric deposition to source pools, yet dissolution of solid phase Ca relative to plant demands may explain why foliar does not exhibit fractionation (Skulan and DePaolo Ca is more sensitive than foliar Mg to patterns of 1999; Gussone and others 2003). We expect that availability in soil. We compared annual Ca and Mg our nitric acid leaching procedure would dissolve inputs from wet deposition (NADP Site OR02, Ca bound by oxalate and other organic compounds, average of 1983–2002 data, http://nadp.sws.uiuc. and we observed 1& d44Ca fractionation in both edu) plus cloud deposition (Weathers and others acid leachable and exchangeable pools relative to 1988) against net annual demands for bole and HF-digests at the soil surface. Such fractionation is branch construction in coastal Oregon Douglas-fir consistent with the idea that depleted d44Ca may (Binkley and others 1992). We estimate that Mg have cycled through oxalate to exchangeable pools deposition (2.1 kg ha)1 y)1) can supply 300% of in surface soils (Figure 4). The enhanced solubility Mg uptake by growing Douglas-fir (0.7 kg ha)1 of Ca-oxalate at low pH has been invoked to ex- y)1). In contrast, Ca deposition (1.6 kg ha)1 y)1) plain long-term increases in Ca export following can supply only about 12% of plant demands harvest of northern hardwood forests (Bailey and (Table 4). Consequently, trees must derive a sig- others 2003). We consider that increased nitrifica- nificant fraction of annual Ca demands from soil tion and lower pH that characterize N-rich soils reserves, which accounts for the close dependency (Table 2) may liberate oxalate bound Ca in similar of foliar Ca on available soil Ca across our study fashion, resulting in the potential for enhanced sites (Figure 2). Given the low rate of atmospheric uptake by plants or accelerated loss via coupled ) Ca input relative to annual plant demands, efficient NO3 and Ca leaching. long-term retention and recycling of Ca between plants and soils is needed to support the high rates Implications for Management of Coastal of primary productivity that are characteristic of this region. Douglas-fir Forests Ca retention in these forests may be promoted by Highly productive coastal Douglas-fir forests of the recycling through sparingly soluble Ca-oxalate Pacific Northwest are capable of storing more C precipitates. The pool of Ca-oxalate in Coast Range (Mg/ha) than any other biome, anywhere on Earth soils is roughly equivalent to exchangeable Ca (Smithwick and others 2002). High soil N avail- (Cromack and others 1979), and a host of soil ability is at least partially responsible for such high invertebrates facilitate Ca-oxalate turnover to productivity, and N is so abundant that fully one- available forms (Cromack and others 1977). Oxa- third of coastal Douglas-fir forests are not N-limited late levels are typically highest in surface soils of (Peterson and Hazard 1990). Foliar N can be used forests (Certini and others 2000) coincident with to identify Douglas-fir stands that are unlikely to oxalate production by roots and fungi. Ionic Ca respond to N fertilization (Hopmans and Chappell bonding (such as occurs with Ca-oxalate) yields 1994), yet N fertilization and interplanting with N2- characteristically depleted d44Ca (<1–3&) relative fixing red alder remain common practices across 72 S. S. Perakis and others

Table 4. Estimated Ca Fluxes and Long-term Supply under Stemwood-only and Whole-tree Harvest

Calcium pool or flux Stemwood harvest Whole-tree harvest

Total Inputs (kg / ha-y) 1.55 1.55 Wet deposition* 0.9 0.9 Cloud deposition¢ 0.6 0.6 Weatheringx 0.05 0.05 Net Uptake and Harvest Removal (kg / ha-y ) )3.2 )13.8 Stemwood only )3.2 )3.2 Branches, twigs, and foliage – )10.6 Net Annual balance (kg / ha-y) )1.65 )12.25 Soil Ca reserves (kg / ha)§ 663 663 Years of available Ca supplyà 402 54

*Average wet deposition recorded at NADP-OR2 for the period 1980–2002. ¢Cloudwater concentrations from Weathers and others 1988, assuming 50 cm/y fog inputs. xPlant-available weathering, set to 3% of total Ca inputs, inferred from 87Sr/86Sr in Figure 3. Data from Binkley and others 1992. Negative sign denotes eventual Ca removal by harvest. §This study, exchangeable Ca 0–65 cm depth. àSoil Ca reserves divided by annual losses. Assumes zero Ca leaching loss and storage in non-harvestable pools. Inclusion of these terms would further diminish Ca supply.

the region, intended to increase forest productivity. Ca is derived from atmospheric inputs, as occurred However, excessive N additions to N-rich forests in the site we sampled for Sr isotopes (Figures 3, 5). have the potential to create nutritional imbalances Although limited in scope, these results support in vegetation, accelerate greenhouse gas (for earlier speculation that weathering may be insuf- example, CH4,N2O) emissions from soils, and im- ficient to support long-term Ca demands of inten- pair downstream water quality (Vitousek and oth- sive forest harvest in the Oregon Coast Range ers 1997). In Oregon Coast Range forests, excess N (Bockheim and Langley-Turnbaugh 1997). has been implicated in the intensification of Swiss Whereas the majority of Coast Range forests are needle cast, an endemic fungal disease that impairs managed by private landowners for timber pro- stomatal conductance and reduces Douglas-fir duction, many federally owned forests are man- productivity by 22–53% (Waring and others 2000; aged primarily for the provision and restoration of Maguire and others 2002; El-Hajj and others 2004). old-growth characteristics within Late-Successional There is great potential to use remote sensing of Reserves. Dense young stands of Douglas-fir cur- foliar and ecosystem N status (for example, Ollinger rently dominate Late-Successional Reserves, yet and others 2002) as a way to identify N sufficient Douglas-fir requires approximately 275 years stands in the Oregon Coast Range, thus maximizing (range = 200–350 years) to develop old-growth returns on fertilization while minimize deleterious characteristics (Franklin and others 2002). Thin- impacts associated with excess N. ning of young dense stands to levels that are typical Rates of nutrient supply from atmospheric of unmanaged forests (for example, Tappeiner and deposition and weathering are an important con- others 1997) is being implemented widely in the straint on the long-term sustainability of managed Pacific Northwest as one way to accelerate re- forests. We estimate in Table 4 the number of years storation of old-growth forests. Typically, thinned of Ca supply that are available to support Douglas- stems are removed and sold to cover the cost of fir growth under two harvest scenarios for condi- thinning operations. On excessively N-rich sites, tions when atmospheric inputs dominate over leaving felled stems onsite could provide benefits weathering as a nutrient source. Under stemwood- by providing a high C:N substrate for N immobili- only harvest in which leaves and branches are left zation, thus reducing nitrate leaching and mitigat- on site, we estimate sufficient Ca is available for ing the potentially deleterious impacts of further approximately 400 years of tree growth. Under ecosystem Ca loss. Remedial lime additions can also whole-tree harvest, we estimate only about 50 restore Ca pools and increase tree growth in young years of Ca supply for tree growth. We expect that stands (M. Gourley, Starker Forest Products, per- real-world harvest operations would remove Ca at sonal communication), and approaches that pro- an intermediate rate due to piling and burning of mote tree species capable of enhancing Ca cycling logging slash. We emphasize that our calculations (for example, western red cedar and big leaf maple; rest on the assumption that 97% of plant-available Tarrant and Issac 1951; Kiilsgaard and others 1987; Coupled Nitrogen and Calcium Cycles 73

Fried and others 1990) may provide a more sus- Bullen TD, White AF, Huntington TG, Peters EN. 1999. A new 87 86 tainable long-term alternative to fertilization. On approach for determining the Sr/ Sr ratio of the granitoid weathering component. In: Armannsson H, Ed. Fifth Inter- sites that are N-rich and Ca-poor (for example, national symposium on geochemistry of the earth’s surface, Figure 2), we suggests that fire may also benefit site Iceland. p 369–72. nutrient balances by promoting N loss through Bullen TD, Bailey SW. 2005. Identifying calcium sources at an combustion, while returning Ca and other mineral acid deposition-impacted spruce forest: a strontium isotope, nutrients to soils in ash. These benefits may not be alkaline earth element multi-tracer approach. Biogeochemis- fully realized in areas where N-fixing red-alder is try 74: 63–90. permitted to reestablish following fire. Capo RC, Stewart BW, Chadwick OA. 1998. Strontium isotopes as tracers of ecosystem processes: theory and methods. Geoderma 82:197–225. ACKNOWLEDGEMENTS Certini G, Corti G, Ugolini FC. 2000. Vertical trends of oxalate We are grateful to Starker Forests, Simpson Timber, concentration in two soils under Abies alba from Tuscany (Italy). J Plant Nutr Soil Sci 163:173–8. the Oregon Department of Forestry, and the Chapin FS, Matson PA, Mooney HA. 2002. Principles of ter- USDA–Forest Service for access to study sites. Jana restrial ecosystem ecology. Berlin Heidelberg New York: Compton and two anonymous reviewers provided Springer. helpful comments on the manuscript. This work Cole DW, Gessel SP. 1992. Fundamentals of tree nutrition. In: was supported by the Swiss Needle Cast Coopera- Chappell HN, Weetman GF, Miller RE, Eds. Forest fertiliza- tive and Cooperative Forest Ecosystem Research tion: sustaining and improving nutrition and growth of wes- programs at Oregon State University, the US Geo- tern forests. Seattle: Institute of Forest Resources Contribution No. 73. p 7–16. logical Survey, and National Science Foundation Compton JE, Church MR, Larned ST, Hogsett WE. 2003. DEB-0346837. Nitrogen export from forested watersheds in the Oregon Coast

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